This invention relates to a process for making products such as paper or tissue from pulp or other fiber-containing material, and more particularly to a process for recovering and recirculating usable fibers contained in water produced in such a process.
The manufacture of products such as paper and tissue uses fibrous material such as wood pulp, which is processed in a known manner to produce the desired end product. In a paper or tissue making process, the pulp is applied to a screen or papermaking fabric from a headbox, and water is pressed out of the pulp in a known manner to form the paper or tissue, which is dried and formed into a roll. The water that is pressed out of the pulp is commonly known as white water, and typically includes small particles of fines and ash material which pass through the fabric along with the water. In addition, the white water inevitably includes a quantity of usable fibers that pass through or around the papermaking fabric, which are wasted if the white water is discarded. This is a recognized problem in the tissue industry, and has resulted in the development of systems that recirculate the white water back into the pulp supply system, to recirculate the usable fibers. However, such systems also recirculate the fines and ash material. This is acceptable in a papermaking process, in which the fines and ash material can be incorporated into the paper. However, the presence of such material is very detrimental in a tissue making process, in that the small particles of material inhibit drainage. Accordingly, simple recirculation systems are undesirable in a tissue making process, since the undesirable fines and ash are simply continuously recirculated in the process. Certain screen systems, which employ a stationary screen, have been developed in an effort to separate the usable fibers from the fines and ash. Typically, fibers retained on the screen are intermittently doctored off the screen and recirculated in the pulp supply system. Because such systems necessarily use screens with small openings, there is a significant tendency for the screen openings to plug or “blind over” due to the buildup of material in the openings. Accordingly, many known systems either do not function properly for this reason, or require a great deal of maintenance to keep the screen openings from plugging.
It is an object of the present invention to provide an effective system for recovering usable fibers from white water in a papermaking system, in order to enable usable fibers to be recirculated into the system without recirculating the undesirable unusable material such as fines and ash commonly found in papermaking white water. It is another object of the invention to provide such as a system which involves little modification to an existing papermaking circulation system, while enabling recovery of usable fibers from white water and recirculating the usable fibers for use. It is a further object of the invention to provide such a system which requires little maintenance and which is relatively simple in its components, construction and operation, to enable the system to be installed and operated at a relatively low cost so as to justify recovery and recirculation of usable fibers from the white water. It is a further object of the invention to replace ineffective existing recovery systems with a recovery system that provides a clean supply of material to the forming fabric to enable more efficient operation of the system.
In accordance with the present invention, a fiber recovery system for a tissue or papermaking process utilizes a filter or screen, onto which white water from the process is directed at a location downstream of a white water collection vessel forming a part of the papermaking system. The screen is sized so as to allow water containing the undesirable or unusable components of the white water, such as fines and ash, to pass through the screen while retaining usable fibers on the screen. The water containing the undesirable or unusable material is routed to a wastewater treatment facility, in a conventional manner, and the cleaned water can then be resupplied to the system. The screen, onto which the white water is directed, is formed of a flexible and pliable screening material, which may be the same type of material as is commonly employed as the fabric in a tissue or papermaking system. The screen is supported in a manner such that the screen is maintained relatively loose and flexible, e.g. by suspending the screen from a frame. The screen is subjected to motion as the white water is directed onto the screen, which results in flexing of the material of the screen, to provide a self-cleaning action of the screen that prevents plugging and blinding of the screen openings. The invention contemplates several different arrangements for supporting and imparting motion to the screen, and for directing the white water onto the screen. In all versions, the white water is applied to an interior area defined by the screen, and the usable fibers are collected on the inner surface of the screen. The screen is configured to direct the usable fibers to an open discharge area, where the usable fibers are discharged from the screen. The usable fibers are then returned to the system and incorporated into the fibrous material supplied to the headbox, for subsequent application to the tissue or papermaking fabric.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.